When measuring the rim do you think its possible that the bevel on the edge of the case was measured rather than the full thickness and that is where the confusion started??? I measured my reformed cases on the edge and they will measure narrower than the the original full rim. I also have a few Kynock cartridges that measure around that .050 + - on the edge of the rim.
The full .303 rim will not feed worth a crap out of the clip unless they are beveled.